store crate-count on repositories, deprioritize bigger workspaces

[syphar]

• store the crate-count on the repositories table. Updated via CLI, or after each build when we finish the release.
• after we processed all changes in the index watcher, we to a bulk activity where we deprioritize the workspaces we find.

caveats:

• this will only work when we actually find the repo in github/gitlab etc. Otherwise we don't have a column in the repositories table. I didn't want to base everything onto the releases.repository column. ( happy to discuss)
• some edge cases depending on where and how you use the build-queue classes. I didn't want the deprioritization logic in process_version_added, because the query is nontrivial.

deploy checklist

• after deploy, one of the repo-stats backfill commands have to be run to the point that the crate-counts are filled initially.
• I can probably also remove some of the hardcoded crate priorities via prefixes.

[GuillaumeGomez]

To be noted: would be nice to have the config documented somewhere.